The present invention relates to a color image forming apparatus of an electrophotographic type and particularly to a compact color image forming apparatus wherein a copy mode is changeable between a color mode and a monochromatic mode.
In an electrophotographic image forming apparatus for forming exclusively monochromatic images such as black images or the like, the diameter of a photoreceptor drum that is an image carrier is required to be small for the purpose of making the apparatus small, and the drum with a small diameter has been put to practical use recently.
Even in an image forming apparatus of a transfer drum type in which a transfer drum is used as an image forming apparatus capable of forming a full color image, there has been known an apparatus employing a photoreceptor drum whose diameter is small.
However, an image forming apparatus of a transfer drum type tends to be large in size because of its transfer drum used therein and to be complicated in structure and expensive in cost because timing adjustment for latent image forming on a photoreceptor drum and for image-transferring onto an image-transfer paper from a transfer drum (rotational synchronization between a photoreceptor drum and a transfer drum) is necessary to assure the registration for an toner image of each color. Besides, a diameter of a photoreceptor drum is made small, which requires a structure of a so-called rotary developing unit wherein only one of plural developing sub-units containing respectively different color toners is located at the position successively to face a photoreceptor drum for developing, which further causes a complicated structure.
As a solution of the aforesaid problems on an image forming apparatus of a transfer drum type, on the other hand, there is known an image forming apparatus wherein no transfer drum is needed and a full color toner image is formed on a photoreceptor drum and then is transferred correctively onto an image-transfer paper or onto a sheet for OHP use.
For the purpose of obtaining a compound image such as a full color image or the like up to B4 size maximum and of making the diameter of a photoreceptor drum small as far as possible, in an image forming apparatus of this type, there has been used a method wherein a compound toner image including a full color toner image is formed through plural turns of a photoreceptor drum (for example, a toner image of each color is formed through one turn of a photoreceptor drum).
When a compound toner image is formed through plural turns of a photoreceptor drum as in the case mentioned above, namely when an entire one toner image is formed on a photoreceptor drum, the size of a toner image formed is restricted by the length in the axial direction and the circumferential length of the photoreceptor drum. In this connection, Japanese Patent Publication Open to Public Inspection No. 229165/1987 (hereinafter referred to as Japanese Patent O.P.I. Publication) has a description saying that the circumferential length on a photoreceptor drum from an image-wise exposure section to a cleaning member is made longer than the length of the maximum copy paper size.
From the aforesaid Japanese Patent O.P.I. Publication No. 229165/1987, it is understood that the maximum size of an image transfer paper is limited when a diameter of a photoreceptor drum is fixed to a certain value and thereby the circumferential length is fixed, while the diameter (circumferential length) of the photoreceptor drum is determined by the desirable maximum size of an image-transfer paper. This merely shows the relation between the circumferential length of a photoreceptor drum and the maximum copy size, and it is nothing but providing an image forming apparatus wherein the selection of one size determines the other size.
Therefore, even in the case of forming a monochromatic image such as, for example, a black toner image produced by a single developing sub-unit or a red toner image produced through developing a latent image formed collectively through the operation of both developing sub-units containing respectively yellow toner and magenta toner, the size of a toner image formed on a photoreceptor drum or the maximum size of an image-transfer paper has been limited by the predetermined circumferential length of a photoreceptor drum, being affected by an image formed by plural turns of a photoreceptor drum such as a full color image or the like.